The present invention relates to semiconductor devices, and particular to semiconductor CCDs, arrays which convert a light field to a spatially resolved discrete electronic charge distribution. In such devices, light striking a semiconductor substrate dislodges electrons resulting in a localized charge region. A gate structure is fabricated adjacent the region, and is operated to periodically sample the accumulated charge. The semiconductor substrate limits the mobility of the dislodged electrons so that the charge distribution gated out of the substrate faithfully reflects the distribution of photons striking the device. A typical device may include an array of several hundred pixels in each direction.
In order to achieve greater light sensitivity in such devices, it is desirable that the device be formed on a very thin semiconductor substrate, i.e., a membrane, and that the illumination field strike the device from the opposite side of the substrate than that on which the gate array structure is fabricated. In devices of this type the light is directed at the back surface of the membrane, and the electrons freed by light in each region must travel to a gate of the array fabricated on the front surface of the membrane.
One factor which impairs the efficiency of such devices is that surface interactions are possible which may allow those freed electrons residing at, or passing near, the back surface to recombine with holes. This effect causes a drop in the detected charge level. Thus it is desirable to prevent electrons dislodged by incoming photons near the rear surface from reaching that surface. In a sensor intended for sensing ultraviolet radiation, for which the absorption coefficient is large and substantially all electron-photon interactions occur in a shallow depth of well under a micron at the rear surface, this effect is particularly pronounced.